A crankcase lubricant is an oil used for general lubrication in an engine where an oil sump is situated below the crankshaft of the engine and to which circulated oil returns. It is well known to include additives in crankcase lubricants for several purposes. Friction modifiers, also referred to as friction-reducing agents, may be boundary additives that operate by lowering friction coefficient and hence improve fuel economy.
Examples of friction modifiers are inorganic friction modifiers in the form of oil-soluble molybdenum compounds, which have been and are used commercially.
Such molybdenum compounds may, for example, be mononuclear, dinuclear or trinuclear according to the number of molybdenum atoms in the molecules of the compounds. The art describes mononuclear molybdenum compounds (sometimes referred to as complexes) and their use in lubricating oil compositions. For example, reference may be made to U.S. Pat. Nos. 4,588,829; 4,889,647; WO-A-2008/092944; WO-A-2008/092945; and WO-A-2008/113814 ('814). All but one of these references describe mononuclear molybdenum compounds where the molybdenum has an oxidation state of greater than +4; '814 is the exception in describing mononuclear molybdenum compounds where the molybdenum has an oxidation state of +4.
'814's molybdenum compounds have the potential energetic advantage of bearing molybdenum in the same oxidation state as MoS2, which is believed to be the species, derived from the complex in use, responsible for the observed beneficial properties of molybdenum complexes in lubricating oil compositions. However, '814's molybdenum compounds contain eight atoms of sulfur per molecule, as seen in General Formula (1) in '814. Modern requirements are to reduce sulfur levels in lubricating oil compositions.